The invention relates generally to controlling the fuel pressure in the fuel system of an internal combustion engine.
Recent trends in motor vehicle fuel efficiency and emission control have increased the temperature of fuel delivered to an internal combustion engine. Elevated temperatures and the corresponding elevation in fuel pressure may result in rapid fuel vaporization, or foaming, when the fuel encounters a sudden pressure drop such as, for example, when entering the fuel bowl of a carburetor.
Fuel foaming in a carburetor has been found to create particularly troublesome problems (see, SAE Technical Paper 821202, entitled "Carburetor Foaming And Its Influence On The Hot Weather Performance Of Motor Vehicles", by V. M. Tertois and B. D. Caddock, 1982). For example, the foam may cause the carburetor float to sink leaving the fuel inlet valve open. Excessive fuel is then forced by the fuel pump into the carburetor bowl and into the engine through both the main jet and the carburetor internal vent. Further, the foam may block the carburetor inlet vent thereby increasing the pressure within the bowl. Any one of the above conditions will likely result in an overfueled engine and associated drivability problems.
Present motor vehicle fuel systems may also underfuel an internal combustion engine. In a carbureted system, for example, the fuel inlet valve has a fixed cross-sectional area. At a constant fuel pump pressure, the volume of fuel flow into the bowl is therefore constant. Accordingly, when a full throttle condition is initiated, the fuel in the bowl may become temporarily depleted thereby underfueling the engine.
A prior approach to regulating the fuel pressure comprised a mechanical pressure valve coupled to the carburetor fuel inlet, and a return fuel line coupled between the fuel tank and the pressure valve. A disadvantage of this approach is that the valve restriction adds to the temperature of the fuel, and also may entrain air within the fuel, thereby increasing the likelihood of fuel foaming at the carburetor bowl. The return fuel line and fuel circulated therethrough also increases fuel temperature.
U.S. Pat. No. 4,260,333 discloses another approach for a fuel injection system wherein the excess fuel not required for injection is returned to the fuel reservoir. The electrical power supplied to the fuel pump is altered continuously in an attempt to maintain the fuel pressure at a single fixed value. A system of this type, however, is inherently prone to oscillations in fuel pressure around the fixed pressure value.
There still remains a need for a fuel system wherein fuel pressure variations which would cause underfueling or overfueling are avoided. Further, a need remains for a fuel system wherein the drivability of a motor vehicle is not subject to rapid variations.